The present invention relates to the electronics arts. It especially relates to flip-chip group III-nitride light emitting diodes for lighting applications, and will be described with particular reference thereto. However, the invention also finds application in conjunction with other types of flip-chip light emitting diodes, and in other optoelectronic devices.
In the flip-chip mounting configuration, a light emitting diode with a light-transmissive substrate and front-side electrodes is bonded “face down” to a substrate. For example, a gallium nitride-based light emitting diode that includes active gallium nitride-based layers grown on a transparent sapphire or silicon carbide substrate can be flip-chip bonded. The flip-chip mounting configuration has a number of advantages, including improved thermal heat sinking due to a close proximity of the front-side active layers to the heat sinking substrate, and elimination of shadowing losses due to the contacts.
The electrodes of the light emitting diode die perform several functions in the flip-chip arrangement, including providing ohmic contacts to the active layers, efficiently reflecting light to contribute to light extraction, and providing thermal pathways for removing heat from the active layers during device operation. The multitasking of the electrodes presents design problems, as it may be difficult to simultaneously optimize optical, electrical, and thermal properties of the electrodes.
In the past, nickel/aluminum (Ni/Al), nickel/silver (Ni/Ag), and other multilayer metal stack electrodes have been used for flip chip group III-nitride light emitting diodes. These electrodes have certain disadvantages. Reflectivity of the metal/semiconductor interface is variable, and depends upon deposition conditions and subsequent processing such as annealing process operations. Metal electrode reflectivity in finished devices is often lower than desired.
While light emitting diodes typically produce light over a narrow spectral range, approximately corresponding to the bandgap of the active semiconductor layer or layers, the reflectivity of metal electrodes is generally only weakly dependent wavelength within the visible wavelength region. This inability to tune electrode reflectivity can be disadvantageous for certain applications, such as fiber optical communications, that benefit from a highly wavelength-selective light output.